The present invention relates to computer networking generally and more specifically to a technique for applying power to a local area network (LAN) device where the power is supplied over the network cable.
The IEEE Standards Association (IEEE-SA) has developed specifications for supplying DC power to network devices over the physical networking cables, as described in the draft IEEE standard P802.3af specifications. This is generally referred to as the Power over Ethernet (PoE) specification. Generally such cables are known as Category 5 cabling and provide network communication links and also DC power to Ethernet data terminals connected to the network. Since this technology enables the powered devices, to receive their operating power over the same Ethernet LAN connection used for data communication, there is no need for each powered device to be connected to an auxiliary AC power socket or an AC/DC power converter.
The technology described in the P802.3af specification is also being updated in a P802.3at specification that is expected to increase the power that can be supplied on each port to a maximum value that is nearly double the initial amount or more than the maximum of 15.6 watts per port, although the maximum value has not yet been determined.
A Power over LAN system comprises an Ethernet switch and a power hub, which serves as the DC power source, along with a number of powered device terminals, which communicate via the switch and draw power from the hub. The system is typically connected in a star topology, with each terminal being linked by a cable to the switch and hub. The power hub in one chassis may be integrated with the switch in a second chassis, in a console containing both chassis, in what is known as an “end-span” configuration. Alternatively, the-power hub chassis may be located between the switch chassis and the terminals, in a “mid-span” configuration. DC power is carried to the loads (i.e., the terminals) over twisted pairs provided by Category 5 cabling. The end-span configuration uses twisted-data-pairs that are also used for Ethernet communication; the mid-span configuration uses spare twisted-spare-pairs that are not used for Ethernet communication.